Keita is a third-year junior-high school student studying for his high school entrance exams and although he is the younger brother, it is obvious he is the most mature of the three siblings. He wants to enter the same high school as his older twin step-sisters Ako and Riko, and to do so must study hard while getting help from his sisters as well as he was originally planning on relying on his track running scholarship for another school. While at first he initially dislikes himself for becoming attracted to Ako and Riko, he later becomes proactive in his desires. It is later revealed he was in love with his sisters when they were kids, and even used all of his money that he saved up for a prayer that he could marry both of them. This changed when he found out from his father that polygamy is illegal and "grew up". He is weak when it comes to alcohol, and will become livelier when he drinks, but after he sobers up he does not remember anything he did. After chapter eighteen of the manga, he makes it into his sisters' school Gakushū Senior High. He has strong morals (that have clearly weakened with the constant "attacks" his sisters have done throughout the series), as he will try to resist to the best he can, but if pushed too far he will simply indulge in the moment. He has also shown an attraction towards Miharu and Yuzuki.